Method and apparatus for subcarrier and antenna selection in mimo-ofdm system

ABSTRACT

A method and apparatus for radio resources control in a multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) communication system are disclosed. A channel metric is calculated for each of a plurality of transmit antennas. Sub-carriers are allocated to each transmit antenna in accordance with the channel metric of each transmit antenna. Signals are transmitted using the allocated sub-carriers at each antenna. Adaptive modulation and coding and transmit power control of each sub-carrier may be further implemented in accordance with the channel metric. Power control may be implemented per antenna basis or per sub-carrier basis. In performing power control, a subset of transmit antennas may be selected and waterpouring may be applied only to the selected antennas. Waterpouring may be based on SNR instead of channel response.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/082,286 filed Mar. 17, 2005, which claims the benefit of U.S.Provisional Application No. 60/601,200 filed Aug. 12, 2004, which isincorporated by reference as if fully set forth.

FIELD OF INVENTION

The present invention is related to a wireless communication system.More particularly, the present invention transmits a data stream usingmultiple antennas in an orthogonal frequency division multiplexing(OFDM) communication system.

BACKGROUND

OFDM is a data transmission scheme where data is split into a pluralityof smaller streams and each stream is transmitted using a sub-carrierwith a smaller bandwidth than the total available transmissionbandwidth. FIG. 1 shows a graphical representation of orthogonalsub-carriers in OFDM. The efficiency of OFDM depends on choosing thesesub-carriers orthogonal to each other. In other words, the sub-carriersdo not interfere with each other while each carrying a portion of thetotal user data.

OFDM system has advantages over other wireless communication systems.When the user data is split into streams carried by differentsub-carriers, the effective data rate on each subcarrier is muchsmaller. Therefore, the symbol duration is much larger. A large symbolduration can tolerate larger delay spreads. In other words, it is notaffected by multipath as severely. Therefore, OFDM symbols can toleratedelay spreads that are typical in other wireless communication systems,and do not require complicated receiver designs to recover frommultipath delay.

As shown in FIG. 2, splitting the data stream into multiple paralleltransmission streams still keeps the basic user data rate the same.Since each symbol duration increases proportionally, any delay spread isproportionally smaller. In practical implementations, the number ofsubcarriers is from 16 to 2,048.

Another advantage of OFDM is that the generation of orthogonalsub-carriers at the transmitter and receiver can be done by usinginverse fast Fourier transform (IFFT) and fast Fourier transform (FFT)engines. Since the IFFT and FFT implementations are well known, OFDM canbe implemented easily and does not require complicated receivers.

FIG. 3 is a block diagram of an exemplary OFDM transmitter and receiver.The heart of the transmitter and receiver are IFFT and FFT blocks. IFFTand FFT operations are mathematically almost the same. Therefore, asingle computation engine is typically used for both IFFT and FFToperations.

For the benefits that OFDM provides, (i.e., simpler implementation,resistance to larger delay spreads, and efficient use of the spectrum),OFDM is one of the preferred wireless transmission schemes today. It isused in WLAN air interface such as 802.11a, WMAN such as 802.16, and itis part of many wireless communication standards.

Multiple-input multiple-output (MIMO) refers to the type of wirelesstransmission and reception scheme where both a transmitter and areceiver employ more than one antenna. FIG. 4 shows such a MIMOtransmitter and receiver. A MIMO system takes advantage of the spatialdiversity or spatial multiplexing and improves signal-to-noise ratio(SNR) and increases throughput.

There are primarily two types of MIMO systems. One type of MIMO systemmaximizes transmission data rate by taking advantage of the paralleltransmissions with MIMO. An example of this type of MIMO scheme is theBLAST system. In this type of a system, the data stream is split intomultiple parallel streams and sent across the air interface in parallel.Using a successive interference canceller (SIC) type detector, thereceiver separates and collects all parallel streams. Therefore, theeffective data rate over the air is increased.

Another type of MIMO system is Space-Time Coding (STC). An STC systemprovides a much more robust link and therefore can support higher signalconstellations. In other words, STC increases the data rate over the airinterface by increasing the signaling order, and therefore increasingthe effective data rate over the air. An example of STC for a 2×2 MIMOis the so called Alamouti codes.

One of the techniques for increasing the efficiency of OFDM is“waterpouring” and refers to the way that the transmit power of eachsub-carrier in OFDM is selected. FIG. 5 shows a typical waterpouringprocess. The transmitter obtains the channel estimation (step 1),inverts it (step 2), and allocates power to the correspondingsub-carriers starting from the lowest point until the total transmitpower is reached (step 3). In order to implement waterpouring, thechannel gain information across the transmission band should be known atthe transmitter. The receiver may send channel estimation informationback to the transmitter in a closed loop manner, or the transmitter caninfer the channel from the signals received from the other side.

FIG. 6 shows a block diagram of a prior art MIMO system, such as VBLAST,where data is converted to parallel and transmitted over multipleantennas.

FIGS. 7 and 8 show a prior art scheme for controlling transmit power ormodulation and coding scheme per antenna basis. In prior art, transmitpower or modulation and coding scheme are determined in accordance withaverage channel gain or other metric. This scheme introduces flexibilityby allowing the transmitter to allocate transmit power or modulation andcoding scheme differently to different antennas based on channelresponse seen at the receiver for each transmit antenna.

FIG. 9 is a block diagram of a prior art system for operation in CDMAbased systems known as PARC (per antenna rate control). This schemetransmits using the full transmit bandwidth from each antenna, astypical of any CDMA system. The prior art systems only address perantenna rate control, and not well suited for OFDM application sincethey are not making use of the sub-carrier level resource allocationavailable in OFDM.

FIG. 10 is a block diagram of another prior art system, called S-PARC(selective per antenna rate control). This scheme transmits using thefull transmit bandwidth from each antenna, as typical of any CDMAsystem.

Prior art systems are not capable of taking advantage of the subcarrierlevel resource allocation that OFDM enables. The prior art systemadjusts transmit power for each antenna transmission according toaverage gain across the band that the receiver sees from each transmitantenna. Therefore, the prior art systems are not suitable for OFDMwhere sub-carrier level resource control is available.

SUMMARY

The present invention is related to a method and apparatus for radioresources control in an MIMO-OFDM system. A channel metric is calculatedfor each of a plurality of transmit antennas. Sub-carriers are allocatedto each transmit antenna in accordance with the channel metric of eachtransmit antenna. Signals are transmitted using the allocatedsub-carriers at each antenna. Adaptive modulation and coding andtransmit power control of each sub-carrier may be further implemented inaccordance with the channel metric. Power control may be implemented perantenna basis or per sub-carrier basis. In performing power control, asubset of transmit antennas may be selected and waterpouring may beapplied only to the selected antennas. Waterpouring may be based on SNRinstead of channel response. With the scheme of sub-carrier levelresource control and power control, an additional dimension offlexibility is provided to optimize the system and increase practicalthroughput and link margin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graphical representation of orthogonal sub-carriers inOFDM.

FIG. 2 is an illustration of splitting data streams into multipleparallel transmission streams.

FIG. 3 is a block diagram of an OFDM transmitter and receiver.

FIG. 4 is a block diagram of a prior art MIMO system.

FIG. 5 shows a waterpouring procedure.

FIG. 6 is a block diagram of a prior art MIMO system.

FIG. 7 is a diagram showing prior art method for controlling transmitpower in MIMO system.

FIG. 8 is a diagram showing prior art method for adaptive modulation andcoding in MIMO system.

FIG. 9 is a block diagram of a transmitter implementing prior art PARCscheme.

FIG. 10 is a block diagram of a transmitter implementing prior artS-PARC scheme.

FIG. 11 is a diagram showing allocation of sub-carriers in accordancewith a first embodiment of the present invention.

FIG. 12 is a block diagram of a transmitter in accordance with the firstembodiment of the present invention.

FIG. 13 is a diagram showing AMC.

FIG. 14 is a block diagram of a waterfilling variation of the presentinvention.

FIG. 15 is a block diagram of a transmitter in accordance with a secondembodiment of the present invention.

FIG. 16 is a diagram showing application of waterpouring technique inaccordance with the present invention.

FIG. 17 is a diagram showing application of SNR-based waterpouringtechnique in accordance with the present invention.

FIG. 18 is a flow diagram of a process for sub-carrier and antennaselection in an MIMO-OFDM communication system in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described with reference to the drawingfigures wherein like numerals represent like elements throughout.

Described below are the preferred embodiments of the present inventionrelating to the use of sub-carrier level resource allocation along withselection of antennas, modulation order, coding scheme, transmit powerlevel, or the like in order to make full use of the capabilities of OFDMand MIMO.

The present invention can be implemented both in a wirelesstransmit/receive unit (WTRU) and a base station. The terminology “WTRU”includes, but is not limited to, a user equipment, a mobile station, afixed or mobile subscriber unit, a pager, or any other type of devicecapable of operating in a wireless environment. The terminology “basestation” includes, but is not limited to, a Node-B, a site controller,an access point or any other type of interfacing device in a wirelessenvironment.

FIG. 11 is a diagram showing sub-carrier level resource allocation inaccordance with a first embodiment of the present invention. Channelmetric is calculated for each transmit antenna and sub-carriers areselected and allocated to each transmit antenna in accordance with thechannel metric. All, a subset or none of sub-carriers are allocated toeach transmit antenna. The set of sub-carriers allocated to eachtransmit antenna may be distinct from one another if such flexibility isdesired or may be the same for simplicity. This gives flexibility tocompromise between better link margin versus better throughput andpermits more flexibility in resource allocation.

Since not all MIMO channels behave the same, but some channels areimpaired or fade more than the others, and channel response is a timevarying behavior and is frequency selective, it is not optimum totransmit all sub-carriers through all antennas. Better overall linkquality can be achieved when a set of sub-carriers are transmitted onchannels having sufficient quality for a constant average transmitpower.

FIG. 12 is a block diagram of a transmitter 100 in accordance with thepresent invention. The transmitter 100 comprises a plurality of antennas102, a serial-to-parallel converter 104, a channel estimator 106, aplurality of sub-carrier modulation units 108, a controller 110 and asub-carrier generation unit 120. A serial input user data stream isconverted by the serial-to-parallel converter 104 to a plurality ofparallel data streams. Each parallel data stream is modulated by arespective sub-carrier modulation unit 108 and forwarded to eachtransmit antenna 102 for transmission.

The channel estimator 106 calculates channel metric from measurements orquality indicators for each of transmit antennas 102. The channel metricmay be estimated by the transmitter 100 in an open loop manner or may bereported from other communicating entity in a closed loop manner. In anopen loop case, the channel estimator 106 performs channel estimationfrom the received signals, and in a closed loop case, a communicationentity that receives communication signals from the transmitter 100performs channel estimation and reports it back to the transmitter 100.

Each data stream is modulated by the sub-carrier modulation unit 108 inaccordance with output signals from the controller 110. The controller110 selects all, subset or none of sub-carriers for each transmitantenna 102 in accordance with the channel metric of each antenna 102.For example, if a channel gain is used as a channel metric, thecontroller 110 selects sub-carriers that exceed a predeterminedthreshold. A different, same or overlapping set of sub-carriers may beallocated to each antenna.

Optionally, the transmitter 100 may further perform adaptive modulationand coding (AMC) and power control per transmit antenna. The transmitter100 includes an AMC unit 112 and/or a gain 114 for each transmit antennaand adjusts modulation order/coding rate and/or transmit power of eachtransmit antenna (per antenna power control) in accordance with thechannel metric of each transmit antenna 102.

FIG. 13 shows a scheme of AMC for each transmit antenna. As shown inFIG. 13, a different modulation order or coding rate may be applied toeach transmit antenna 102 in accordance with channel metric of eachtransmit antenna 102. The AMC unit 112 adjusts modulation order and/orcoding rate applied to data stream for each transmit antenna 102 inaccordance with control signal from the controller 110.

The transmit power level for each transmit antenna is adjusted at thegain device 114 in accordance with control signal from the controller110. The transmit power control may be either open loop or closed loop.

FIG. 14 is a block diagram of another embodiment of the presentinvention. The transmitter 100 further includes a mapper 118 for mappingeach data stream to a transmit antenna 102 in addition to the elementsof the first embodiment. The mapper 118 selects a transmit antenna 102and cross-connects each data stream to a transmit antenna 102 inaccordance with the control signal from the controller 110.

In this embodiment, the MIMO scheme selects one or more antennas fortransmission based on a metric that is calculated using measurements andquality indicators reported or estimated, and in addition the schemeselects all available subcarriers or a subset of them for transmission.In other words, the scheme selects the combination of best antenna, or aset of antennas, and subcarriers. Note that subsets or subcarriers maybe distinct for each antenna if such flexibility is desired orconstrained to be the same for simplicity sake. This gives the operatorof the system flexibility to compromise between better link marginversus better throughput and permits more flexibility in resourceallocation during scheduling. Both open loop or closed loop schemes maybe used.

Not all MIMO channels behave the same, some are impaired or fade morethan the others or exhibit unfavorable correlation to other channels.This behavior is a time varying and frequency selective behavior.Therefore, it is not optimum to transmit all subcarriers in allchannels. Better link quality can be achieved when a set of subcarriersare transmitted on better quality channels for a constant averagetransit power within any applicable power spectral density requirements.

This embodiment recognizes that the quality of each MIMO channel andeach OFDM subcarrier (channel) will in general be different and timevarying, and that a diversity/capacity advantage can be gained byintelligent usage of those channels. The channel qualities may besignaled to, or estimated by, the transmitter. Complexity andregulations in some implementations may limit the antenna/frequencyflexibility.

FIG. 15 is a block diagram of a transmitter 100 in accordance with asecond embodiment of the present invention. The transmitter 100 maycomprise a plurality of transmit antenna circuits, each transmit antennacircuit including an AMC unit 112, a subcarrier modulation unit 108, asub-carrier transmit power control (TPC) unit 116 and a transmit antenna102. The transmitter 100 may further comprise the serial-to-parallelconverter 104, the channel estimator 106 and the controller 110 used inthe first embodiment. The channel estimator 106 may calculate a channelmetric for each of the transmit antenna circuits. The controller 110 isin communication with the channel estimator 106 and the transmit antennacircuits, and may be used to select a subset of the transmit antennacircuits. The controller 110 may be further used to determine whether toallocate all, a subset or none of a plurality of OFDM sub-carriers foreach of the selected transmit antenna circuits based on the channelmetrics calculated by the channel estimator 106. The selected transmitantenna circuits may be used to adjust transmit power for each of theallocated OFDM sub-carriers, and to transmit signals using the OFDMsub-carriers.

The sub-carrier TPC unit 116 in each of the transmit antenna circuitsadjusts transmit power level for each sub-carrier in accordance with acontrol signal received from the controller 110. Sub-carrier leveltransmit power control is, preferably, a waterpouring technique,although other techniques may be used. The transmit power level of eachsub-carrier is adjusted according to the channel response for eachsub-carrier. Therefore, the transmit power level across the transmissionband is different for each sub-carrier or groups of sub-carriers.

The waterpouring algorithm preferably operates across all antennas andall subcarriers and adjusts the transmit power level for eachsubcarrier. However, this is sometimes not desirable. When a full set ofN transmit antennas and M receive antennas are used, the complexity ofthe receiver is typically proportional to M⁴N⁴. In other words, thecomplexity of the receiver is affected by the number of antennas at thetransmitter and the receiver. Moreover, it is often the case that notall antenna signals go through desirable channel conditions.

Another embodiment of the present invention is an enhancement to theaforementioned waterpouring technique. In accordance with thisembodiment, a subset of transmit antenna is selected for transmissionand waterpouring is applied only to the selected transmit antenna(s).FIG. 16 shows selection of a transmit antenna in accordance with channelresponse of each transmit antenna. In FIG. 16, antennas 102 _(a), 102_(c), 102 _(d) are selected for transmission and antenna 102 _(b) isexcluded from transmission. After a subset of antennas, such as antennas102 _(a), 102 _(c), 102 _(d), is selected, a waterpouring technique, oralternative technique may be applied to the selected transmit antennas.

The number of transmit antennas 102 is maintained at a reasonablenumber, (may be predetermined), and keeps the receiver complexity down.At the same time, by selecting the best antenna combination overallperformance is maintained.

In accordance with another embodiment of the present invention,waterpouring is implemented based on SNR, instead of channel response.This technique considers the impact of the noise level present at eachsub-carrier. Typically, the background noise is treated as being white.In other words, the background noise is assumed to be the same level forall sub-carriers. This assumption is typically not correct forunlicensed bands. In unlicensed bands, other transmissions can overlapwith part of the sub-carriers in the transmission band and the receivedsignal may be subject to substantially different levels of interferenceregardless of the channel response. Therefore, SNR can provide a bettermetric for each sub-carrier or group of sub-carriers, although otherinterference/noise/signal measurements may be used, such as signal tointerference ratio (SIR) or signal to interference noise ratio (SINR).The background noise level can be substantially different for differentpart of spectrum, and hence the preferred solution may be different thanthe one that assumes a flat noise spectrum.

FIG. 17 illustrates a scheme of SNR-based waterpouring. FIG. 17 showsboth channel response and SNR for the OFDM spectrum. Sub-carriers areselected and transmit power is allocated in accordance with the SNR.Compared to FIG. 16, which waterpouring is based on channel response, inFIG. 17, some of the subcarriers are newly added and some are removed.This embodiment better accommodates and preserves high performance incases where the background noise level across the spectrum is changingin addition to the channel response.

FIG. 18 is a flow diagram of a process 200 for sub-carrier and antennaselection in an MIMO-OFDM communication system. Channel metric for eachof a plurality of transmit antennas is obtained (step 202). Sub-carriersare allocated to each antenna in accordance with the channel metric ofeach antenna (step 204). Messages are transmitted using the allocatedsub-carriers at each antenna (step 206). AMC may be performed perantenna basis, per sub-carrier or group of sub-carrier basis. Powercontrol may be implemented per antenna basis or per sub-carrier basis.In performing power control per sub-carrier basis, a subset of transmitantennas may be selected and waterpouring may be applied only to theselected antennas. Waterpouring may be based on SNR instead of channelresponse.

The elements of FIGS. 12, 14 and 15 may be implemented using a singleintegrated circuit (IC), such as an application specific integratedcircuit (ASIC), multiple ICs, discrete compoments or a combination ofIC(s) and discrete components.

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the preferred embodiments or in various combinations with orwithout other features and elements of the present invention.

1. A method, implemented by a transmitter, of selecting antenna circuitsfor transmitting signals, the method comprising: configuring thetransmitter with a plurality of antenna circuits, each antenna circuitincluding a sub-carrier modulation unit, a sub-carrier transmit powercontrol (TPC) unit and an antenna connected in series; using a channelestimator residing in the transmitter to calculate a channel metric foreach of the antenna circuits; and using a controller residing in thetransmitter and in communication with the channel estimator and theantenna circuits to select a subset of the antenna circuits.
 2. Themethod of claim 1 further comprising: using the controller to determinewhether to allocate all, a subset or none of a plurality of orthogonalfrequency division multiplexing (OFDM) sub-carriers for each of theselected antenna circuits based on the channel metrics calculated by thechannel estimator.
 3. The method of claim 2 further comprising: usingthe selected antenna circuits to adjust transmit power for each of theallocated OFDM sub-carriers; and using the selected antenna circuits totransmit signals using the allocated OFDM sub-carriers.
 4. The method ofclaim 2 further comprising mapping the OFDM sub-carriers to at least oneof the antennas.
 5. The method of claim 2 further comprising performingadaptive modulation and coding (AMC) on a per-OFDM sub-carrier basis. 6.The method of claim 1 wherein the transmitter is located in a wirelesstransmit/receive unit (WTRU).
 7. The method of claim 1 wherein eachantenna circuit further includes an adaptive modulation and coding (AMC)unit.
 8. A transmitter comprising: a plurality of antenna circuits, eachantenna circuit including a sub-carrier modulation unit, a sub-carriertransmit power control (TPC) unit and an antenna connected in series; achannel estimator configured to calculate a channel metric for each ofthe antenna circuits; and a controller in communication with the channelestimator and the antenna circuits, the controller configured to selecta subset of the antenna circuits.
 9. The transmitter of claim 8 whereinthe controller is further configured to determine whether to allocateall, a subset or none of a plurality of orthogonal frequency divisionmultiplexing (OFDM) sub-carriers for each of the selected antennacircuits based on the channel metrics calculated by the channelestimator.
 10. The transmitter of claim 9 wherein the selected antennacircuits are configured to adjust transmit power for each of theallocated OFDM sub-carriers, and to transmit signals using the allocatedOFDM sub-carriers.
 11. The transmitter of claim 9 further comprising amapping unit configured to map the OFDM sub-carriers to at least one ofthe antennas.
 12. The transmitter of claim 9 wherein the controller isfurther configured to perform adaptive modulation and coding (AMC) on aper-OFDM sub-carrier basis.
 13. The transmitter of claim 8 wherein thetransmitter is located in a wireless transmit/receive unit (WTRU). 14.The transmitter of claim 8 wherein each antenna circuit further includesan adaptive modulation and coding (AMC) unit.